1. Field of the Invention
This invention relates to a chemical vapor deposition (CVD) apparatus, more particularly to a loading lock for maintaining the air-tight state of the reaction chamber of such an apparatus.
2. Prior Art Statement
The well-known upright low-pressure chemical vapor deposition (LPCVD) apparatus today constitutes a highly important piece of equipment in the semiconductor production process. This apparatus is typically of the structure shown in FIG. 7. As illustrated, the reaction chamber 2A of the apparatus is formed as a space enclosed by a quartz tube 2 which in turn enclosed within a peripheral heater 1. The quartz tube 2 is supported on stainless steel manifold 6, via a vacuum seal O-ring 5A. The manifold 6 is mounted on a stainless steel support plate 9, via a seal 5B. A quartz carrier 4 for holding semiconductor substrates 3 is supported by a stainless steel seal plate 10 directly connected with a lifting support 11 so that the seal plate 10 can be moved vertically by a lifting mechanism 11'. When raised to its upper limit, the seal plate 10 comes in contact with a seal 5C so as to establish a vacuum seal between itself and the support plate 9. The vacuum chamber defined by the quartz tube 2, manifold 6 and seal plate 10 is evacuated by an evacuator 7' via an evacuation port 7. During the deposition process the vacuum chamber is ordinarily maintained at a pressure of about 0.5 Torr and reaction gas is supplied thereto through a gas feed port 8. After the semiconductor substrates 3 are sufficiently heated up by the peripheral heater 1, the reaction gas atmosphere is established in the reaction chamber properly, giving rise to a thermochemical reaction that produces various types of thin films on the surfaces of the semiconductor substrates 3.
Another conventional apparatus having two quartz tubes disposed one inside the other is also known. Since the manner in which deposition is conducted in this apparatus is essentially the same as that in the single-tube type apparatus shown in FIG. 7, however, it will not be explained further here.
In the upright LPCVD apparatus of the foregoing structure, the reaction chamber has to be once restored to atmospheric pressure before the quartz carrier loaded with silicon substrates or the like is loaded into the reaction chamber and then again before it is removed therefrom after completion of the deposition processing. As a result, atmospheric air is allowed to enter the reaction chamber before and after deposition processing and, moreover, the substrates or the like are directly exposed to atmospheric air while still at a high temperature after processing. This causes the following three problems.
First, at the time of being removed from the reaction chamber the surfaces of the substrates or the like, which are still at a high temperature, are oxidized by atmospheric air and moisture. Second, reaction material deposited on the inner wall of the quartz tube (the reaction chamber) is similarly oxidized and becomes a cause for the release of particles into the chamber atmosphere. Third, air remaining in the reaction chamber at the time the substrates and quartz carrier are loaded (inserted) causes the surfaces of the substrates to be oxidized before the start of the deposition process.
For overcoming these problems, it has been proposed to use a double-chamber system in which a loading lock is constituted by providing an outer chamber in addition to the reaction chamber. A typical example of this arrangement is shown in FIG. 8. As illustrated, the reaction chamber 2' formed within a quartz tube 2" is configured in the same manner as that in the apparatus shown in FIG. 7. Differently from the earlier arrangement, however, a gate valve 22 is provided immediately beneath the manifold 6" and a vacuum chamber 24 is provided below the gate valve 22, via a vacuum seal 23. The interior of the vacuum chamber 24 constitutes an outer chamber 24' which is evacuated to a vacuum state through an evacuation port 25 connected with an evacuator 25'. In the illustrated arrangement, the reaction chamber 2' and the outer chamber 24' can be sealed off from each other by the gate valve 22. When the substrates are to be removed from the reaction chamber 2' upon completion of deposition processing, a lifting mechanism 26 is operated to lower the quartz carrier 4" to the lowered position 27 indicted by chain lines in the drawing, whereafter the gate valve 22 is closed to seal off the reaction chamber 2'. The outer chamber 24' is then restored to atmospheric pressure and a door 28 is opened to allow the substrates to be taken out.
Introduction of the quartz carrier 4" into the reaction chamber 2' after substrates have been mounted thereon is carried out by closing the outer chamber door 28, evacuating the outer chamber 24' to a sufficient degree of vacuum, opening the gate valve 22, and operating the lifting mechanism 26 to raise the seal plate 10".
Even this double-chamber loading lock system, however, has the following problems.
One is that the outer chamber has to be once returned to atmospheric pressure during loading and unloading of the substrates. Even though the outer chamber is evacuated again after introduction of the substrates, a certain amount of residual air unavoidably remains. By way of example, assume that air molecules are present in atmospheric air at an average of 2.68.times.10.sup.19 molecules/cm.sup.3. In this case even if a high vacuum on the order of 10.sup.-6 is achieved, air molecules will still remain in the outer chamber on the order of 10.sup.10 molecules/cm.sup.3. It is thus impossible to prevent oxidation of the substrate surfaces by the residual air.
Another is that the lifting mechanism is located inside the outer chamber so that metallic contaminants, particles and the like from this mechanism will possibly adhere to the substrate surfaces.